NL1011490C2 - Process for desulfurizing gases. - Google Patents

Description

Process for desulfurizing gases

The present invention relates to a method for removing sulfur compounds from a gas stream. In this process, the sulfur compounds are first converted to hydrogen sulfide, after which the hydrogen sulfide in an aqueous solution is converted to elemental sulfur by biological oxidation.

Sulfur compounds are one of the most important pollutants that can occur in waste gases for the oil and gas (processing) industry. It is a contaminant that occurs in high concentrations and the legislation regarding these compounds is very strict.

Therefore, many processes are known to remove sulfur compounds from gases. One of the most important processes is the catalytic conversion of the sulfur compounds to elemental sulfur. A major advantage of this process is that the elemental sulfur is a product with its own economic value.

The most important process for converting sulfur compounds, in particular hydrogen sulfide, into elemental sulfur is the so-called Claus process. With this process, a total sulfur removal of about 95% can be achieved. The remaining amount of sulfur is contained in the so-called Clausaf gas (also referred to as "Claus tail gas" or "Claus 20 off gas") in the form of sulfur compounds such as COS, CS2, SO2 and SO3, but also small amounts of gaseous elemental sulfur (Sx ) and mercaptans (RSH)

Sulfur-containing gas streams can also be formed in other processes, such as synthesis gas or fuel gas, which still contain the aforementioned undesired sulfur compounds. Often these sulfur compounds are converted into hydrogen sulfide by catalytic hydrogenation or catalytic hydrolysis.

An example of the catalytic hydrogenation of these compounds to hydrogen sulfide is the so-called SCOT process (Shell Claus Off-gas Treatment). In this process, the sulfur compounds present in the off-gas from the Claus process are usually converted, after selective removal by an amine-containing solution, the hydrogen sulfide thus formed is returned to the Claus reactor to increase its efficiency.

The waste gas from this SCOT process may also still contain traces of sulfur components, generally traces of COS and CS2. In general, to prevent emission of these compounds, this process will include a post-combustion step, converting these compounds to SO2.

Also, the undesired sulfur compounds can be converted to hydrogen sulfide by catalytic hydrolysis. Such a process is in particular applied to gas streams containing carbonyl sulfide (COS). Typical gas phase hydrolysis catalysts are based on copper sulfide, chromium oxide, chromium oxide alumina and platinum.

Another example of a process in which sulfur compounds present in the Clausaf gas are converted into hydrogen sulfide is the so-called Beavon process. With this process, sulfur compounds are removed from off-gas from the Claus process by hydrolysis and hydrogenation over a cobalt molybdate catalyst, resulting in the conversion of carbonyl sulfide, carbon disulfide and other sulfur compounds to hydrogen sulfide.

The object of the present invention is to provide a process for the conversion of hydrogen sulfide to elemental sulfur, using aerobic bacteria. As a result, the H2S formed from the catalytic reduction step will not be recycled to the Claus reactor. This application is particularly interesting when such a Claus installation is not available, as is the case with so-called stand-alone SCOT units.

A further object of the present invention is to carry out this circumvention in such a way that a high efficiency is thereby obtained.

A process has now been found for removing hydrogen sulfide from a gas stream, washing the hydrogen sulfide with an aqueous solution from the gas phase, biologically oxidizing the hydrogen sulfide in the aqueous solution in a bioreactor to elemental sulfur and the elemental sulfur from the aqueous solution. separates, characterized in that the gas stream to be treated is cooled to such an extent that sufficient water vapor condenses from this gas stream to compensate for the blowdown stream for the removal of salts, so that no water needs to be supplied to the bioreactor.

The method of the present invention offers the following advantages:

With this method elemental sulfur can be obtained with a high efficiency from gas sulphide-containing gas streams.

Any HCN present in the gas stream reacts with elemental sulfur to form thiocyanate (SCN) that is biodegradable.

Traces of other sulfur compounds still in the gas flow 1011 ê § Qj 3 are also converted.

- The process has low energy consumption.

- No expensive chemicals are needed.

- The process is easy to operate.

5

To clarify the present method, the following figures are included:

Figure 1 showing the rate of decrease of the biologically formed sulfur as a function of pH and temperature.

Figure 2 showing an embodiment of the present invention.

It is preferred when the hydrogen sulfide is obtained by catalytic conversion of sulfur compounds.

The sulfur compounds are preferably converted to hydrogen sulfide by catalytic hydrogenation. This conversion is particularly suitable when the sulfur compounds include sulfur dioxide (SO2), sulfur trioxide (SCh), carbonyl sulfide (COS), carbon disulfide (CS2), and sulfur vapor (Sx). Preferably, the sulfur compounds are converted into hydrogen sulfide by means of the hydrogenation step in the above-described SCOT process. It is also possible to convert the sulfur compounds into hydrogen sulfide by catalytic hydrolysis. Catalytic hydrolysis is particularly suitable when the gas stream comprises carbonyl sulfide (COS) and optionally carbon disulfide (CS2) and mercaptans (RSH).

Biological oxidation of hydrogen sulfide to elemental sulfur is known. Such methods are described, for example, in WO 96/30110 and WO 92/10270.

As described above, the hydrogen sulfide is washed from the gas phase with an aqueous solution. This step can be carried out in a scrubber in which an intensive contact between the gas flow and the washing liquid is established

If desired, the wash liquor can be buffered to a µl between 6.0 and 10.0.

The buffering compounds must be tolerated by the bacteria present in the oxidation reactor. Preferred buffering compounds are carbonates, bicarbonates, phosphates and mixtures thereof, in particular sodium carbonate and / or sodium bicarbonate. The concentration of the buffering compounds is generally adjusted to a value between 20 and 2000 meq / l. When sodium carbonate is used as the 1011490 4 buffering compound, the concentration thereof is preferably adjusted to about 15 to 25 g / l. When bicarbonate and carbonate concentrations are used in this specification, these are expressed as the weight concentration of the ions HCCb and CCb-, respectively. The ratio HCO3 to CO3 depends on the pH of the solution which in turn is determined by the partial voltage of CO2 and H2S of the gas stream to be treated.

The addition of buffering compounds can take place after the washing liquid has left the scrubber, but also before it is introduced into the scrubber.

Depending on the type of catalyst, the gas to be desulphurized has a temperature of 160 to 320 ° C. A typical value of the dew point of the gas is between 65 and 75 ° C. The gas then contains about 30% by volume of water. It is necessary to cool the moist gas so that the desired temperature in the bioreactor is reached. The desired equilibrium temperature of the suspension in the bioreactor depends on (1) the temperature at which the micro-organisms are still active and (2) the chemical stability of the sulfur formed. Laboratory research has established that Thiobacilli are capable of oxidizing the sulfide up to 70 ° C. At these high temperatures, however, the sulfur formed will hydrolyze to a significant extent, according to the following reaction equation: 20 (1) 4 S ° + 4 OH - »2 HS '+ S2O32' + H20,

In addition, it is also possible that sulfide (HS) and sulfate (SO-i2) are formed, according to:

25 (2) 4 S ° + 5 OH '3 HS' + SO42 '+ H2O

Laboratory experiments performed with biologically formed sulfur from a THIOPAQ reactor have shown that mainly sulfide (HS) and thiosulfate (S2O? 2) are formed, so that it is assumed that mainly reaction (1) occurs

Figure 1 shows the decrease rate of the biologically formed sulfur as a function of pH and temperature

Since the chemical stability of the sulfur formed decreases with increasing pH and temperatures at 101 1490, the temperature of the suspension in the bioreactor should not exceed 65 ° C.

By cooling the gas below its dew point, part of the water vapor present will condense. This condensation water is used to remove sulphate. This is formed as a result of the undesired oxidation of sulfur, according to: S ° + 1.5 02 + H20 -> 2 H + SO42 '

The sulfate production amounts to about 3 to 10% of the total sulfide load. The essence of the patent application is based on cooling the sour gas below its dew point such that just enough condensation water is formed to compensate for the blowdown.

As bacteria which oxidize sulfur to elemental sulfur in the presence of oxygen in the presence of oxygen (herein referred to as sulfur oxidizing bacteria), suitable autotrophic aerobic cultures, such as of the genera Thiobacillus and Thiomicroshvra, are suitable for this purpose.

It is advantageous if the specific conductivity of the aqueous solution in which the hydrogen sulfide is absorbed is kept constant. The specific conductivity is a measure of the total amount of dissolved salts. This mainly concerns sodium (bi) carbonate and sodium sulfate. The specific conductivity should be controlled within 10 and 100 mS / cm, preferably between 40 and 70 mS / cm.

The amount of oxygen supplied to the washing liquid is controlled so that the oxidation of the absorbed sulphide produces mainly elemental sulfur. Such a method for the controlled oxidation of waste water containing sulfur is described in Dutch patent application 8801009.

The formation of sulfur in the oxidation reactor results in a sulfur slurry which is drained. The sulfur from this suspension is separated from the aqueous solution by filtration, centrifugation, flocculation, settling, etc. After separation, the sulfur can be further worked up by drying and optionally purifying, and reused. The remaining liquid can be reused as a washing liquid.

It has been found to be advantageous if one does not drain all the sulfur, and thus drain the batch discontinuously or in part, so that a washing liquid still contains sulfur. The sulfur concentration in the washing liquid is generally kept between 0.1 and 50, preferably between 1 and 50 and more preferably between 5 and 50 g / l (1 to 5% by weight). in particular, the percentage of sulfur separation is regulated in such a way that as much washing liquid as possible is reused. The liquid recovered when working up the drained sulfur can optionally be added to the washing liquid.

In addition to hydrogen sulfide, the gas may also contain hydrocyanic acid gas (HCN). Particularly in the case of the presence of HCN as a component in the gas, elemental sulfur in the washing liquid is useful. The cyanide, which is poisonous to most bacteria, is thus converted into the much less poisonous thiocyanate, which is subsequently broken down biologically and / or chemically. Ultimately, HCN is converted into carbon dioxide and nitrate. The sulfide concentration in the washing liquid used with a pH of about 8.5, expressed as sulfur, will usually be about 15-3000 mg / l when cleaning gases with about atmospheric pressure.

The ratio between the amount of washing liquid and gas is determined on the one hand by the absorption capacity of the washing liquid for H 2 S, and on the other hand by hydrodynamic characteristics of the scrubber.

The scrubbers to be used according to the invention can be of a conventional type, as long as an effective contact between the gas flow and the washing liquid is established in the scrubbers.

For the process according to the invention, it is preferred, in particular for the aerobic reactor (s), reactors of the vertical circulating type, as described, for example, in International patent application 94/29227, in which the gas to be used (in the aerobic reactor is that usually air can provide vertical circulation

Figure 2 shows a possible embodiment of the method according to the present invention, in which the hot gas is cooled in a quench I 25 combined with a cooler 2, in which a second cooler 3 is placed between the gas scrubber (= absorber) 4 and the THIOPAQ bioreactor 5. The water that condenses in quench 1 is fed to the bioreactor 5.

The gas stream to be treated is fed to quench 1 at 7. In the quench 1 with cooler 2, the temperature of the gas drops below the dew point (65-75 ° C) 30 causing condensation to occur. The condensate goes via line 8 to the bioreactor 5 to serve as make-up water. A part of the cooled gas goes via line 9 to gas scrubber 4. Another part is returned via line 10 to quench 1. From gas scrubber 4 a gas flow 11 and a wet gas flow 12 occur in the case that the 101 1490 7 wet gas is still too warm, so that the temperature in the bioreactor can become too high, a second cooler 3 is mounted between the scrubber and the bioreactor.

From the bioreactor 5, a sulfur stream 13 is fed to the sulfur recovery 6, a blowdown stream 14 and a recycling stream 15 which is fed to the scrubber 5. From the sulfur recovery 6, part of the recovered sulfur is returned via line 16 to the bioreactor 5.

The water content in the hot gas is on average about 33 mol%, which is more than sufficient. The amount of water that condenses can be controlled by adjusting the temperature in the coolers. The colder the set temperature, the more water 10 condenses.

Examples

In the table below, two gas streams are used as an example. These gas streams are treated catalytically before H2S is removed and biologically converted to elemental sulfur.

Table 1: Gas flows as an example for treatment in a catalytic biological desulfurization process.

Π [2

Gas type Claus-off gas Syn gas

Composition (drying gas analysis) (in vol.%) H2S 0.45 1.9 SO2 0.22 COS 75 ppm 0.1 CS2 60 ppm 300 ppm CH? SH - 50 ppm

Sx 600 ppm CO 0.22 29 CO2 3 26 H2 2.2 40 N2 94 0.9 CH4 - 2 101 1490 8

Claus off gas

After catalytic conversion, Claus-off gas (20,000 NniVh) has a temperature of 200 ° C. Approximately 4 tons of sulfur are removed as H2S per day. The hot gas contains 33 vol.% Water vapor. In the Thiopaq reactor, 25 kg SOVhr will be formed, assuming 5% by oxidation. At a typical sulfate concentration in the bioreactor of 18 kg SCVm3, the size of the blowdown flow becomes 1.4 m3 / h. Cooling the gas down to 63 ° C will produce the same amount of condensation water. The cooling is done as follows: By means of a quench the gas is cooled to its dew point of approx. 70 ° C. The biological system is operated at 50 ° C, causing the temperature of the gas in the absorber to drop to 63 ° C. The used wash water will heat up so that a heat exchanger is necessary.

15 Svn gas

After catalytic conversion, it has a syn gas from a temperature of 160 ° C and a flow rate of 6,000 Nm3 / h. Approximately 4 tons of S are removed as H2S per day. The gas contains about 30% by volume of water vapor

To maintain the conductivity at the desired level, a purge flow of 1.4 m3 / h is required. Since the amount of gas is much less than in the previous example, the gas is cooled directly in the absorber. By cooling the gas to 56 ° C, sufficient water vapor condenses to obtain the desired purge flow. The cooling is realized by operating the biological system at 48 ° C. The heated wash water is cooled by means of a heat exchanger.

101 1490

Claims (10)

1. A process for removing hydrogen sulfide from a gas stream, washing the hydrogen sulfide with an aqueous solution from the gas phase, biologically oxidizing the hydrogen sulfide in the aqueous solution in a bioreactor to elemental sulfur and separating the elemental sulfur from the aqueous solution, characterized in that the gas stream to be treated is cooled to such an extent that sufficient water vapor condenses from this gas stream to compensate for the blowdown stream for the removal of salts, so that no water has to be supplied to the bioreactor. The process according to claim 1, wherein the washing out of hydrogen sulfide is carried out under process conditions such that the exiting washing liquid in which the hydrogen sulfide is absorbed is suitable for biological oxidation. 15 A method according to claim 1 or 2, wherein the conductivity in the aqueous solution is kept constant by condensing water. 4 Cf 0Π 4. Process according to any of the preceding claims, wherein the hydrogen sulfide is obtained by catalytic conversion of sulfur compounds. The process according to claim 4, wherein the sulfur compounds are converted by catalytic hydrogenation. The method of claim 5, wherein the sulfur compounds to be removed include sulfur dioxide (SCh), sulfur trioxide (SCh), carbonyl sulfide (COS), carbon disulfide (CS.)), Mercaptans (RSH) and / or sulfur vapor (Sx). 7. Process according to claim 4, wherein the sulfur compounds are converted by catalytic hydrolysis. The method of claim 7, wherein the sulfur compounds to be removed comprise carbonyl sulfide (COS), carbon disulfide (CS2) and mercaptans (R-SH). 1 The process according to any one of the preceding claims, wherein the aqueous solution from which elemental sulfur is separated is recycled. A method according to any one of the preceding claims, wherein the temperature of the bioreactor is kept at a value no higher than 65 ° C. 101 1490